Electric high voltage generators

ABSTRACT

A high voltage generator of the Cockcroft-Walton type is constructed from modular units built up into a stack, the number of units determining the voltage. Each unit is contained within a screen with large radius outwardly facing surfaces. The units are ready stressed internally and the screens ease the stressing problems of building up a stack. A driving inverter of the switched thyristor type has inductance in the overflow diode path so that the inverter will withstand a short across the output and to improve the speed of thyristor recovery.

United States Patent Inventor Alan Davies Wantage, England Appl. No.53,389 Filed July 9, 1970 Patented Oct. 19, 1971 Assignee United KingdomAtomic Energy Authority London, England Priority July 18, 1969 GreatBritain 36418/69 ELECTRIC HIGH VOLTAGE GENERATORS 8 Claims, 11 DrawingFigs. US. Cl 321/8 R, 321/15 Int. Cl H02m 7/00 Field of Search 321/8, 15

References Cited UNITED STATES PATENTS 2/1959 Cleland 321/15 3,063,00011/1962 Cleland 321/15 3,246,230 4/1966 Cleland 321/15 3,303,406 2/1967Bedford 321/44 3,406,328 10/1968 Studtmann 321/45 3,474,320 10/1969Chandler 321/45 C 3,533,010 10/1970 Bowles 321/15 X 3,539,903 11/1970Goebel 321/15 Primary Examiner-William M. Shoop, Jr. Attorney-Larson,Taylor and Hinds ABSTRACT: A high voltage generator of theCockcroft-Walton type is constructed from modular units built up into astack, the number of units determining the voltage. Each unit iscontained within a screen with large radius outwardly facing surfaces.The units are ready stressed internally and the screens ease thestressing problems of building up a stack. A driving inverter of theswitched thyristor type has inductance in the overflow diode path sothat the inverter will withstand a short across the output and toimprove the speed of thyristor recovery.

PATENTEBUBT 19 I9?! 3,614. 588 SHEET 30F 5 4 1121/; v AIM/MA SHEET 5 or5 PAIENTEnum 19 Ian BACKGROUND OF THE INVENTION The invention relates toelectric high voltage generators and more to generators of theCockcroft-Walton type.

A Cockcroft-Walton generator comprises two banks of sefies-connectedcapacitors with unidirectional zig-zag cross connection between thebanks so that the application of an alternating current across the endsof the two banks causes successive charge transfer, with voltageaddition, from the capacitor of one bank to the next capacitor of theother bank and so on.

SUMMARY OF THE INVENTION The invention provides a C4for a high voltagegenerator of the Cockcrott-Walton type, comprising one or moreCockcroft-Walton stages mounted within an electrically conductingcasing, which forms an encompassing screen for the said stages, andinput and output terminals adapted for ready connection, if desired, toanother modular unit so that the voltage setup of each unit is added,whereby a stack of N interconnected modular units can generate a voltageNV, where V is the voltage generated by each modular unit on its own.

By Cockcroft-Walton stage is meant two capacitors with unidirectionalpath connection so that, with an alternating current input, charge istransferred with voltage addition from one capacitor to the other.

It is an important feature of the invention that the said screen is heldat a potential intermediate, preferably midway, between, that of theinput and output terminals of the modular unit.

With this arrangement, the said stages within each modular unit can bestressed for predetermined potential differences between its input andoutput terminals and between these terminals and the encompassingscreen. Each modular unit with its encompassing screen can be stressedfor a predetennined potential difference between it and an adjacentmodular unit in a stack of modular units.

By stressing is meant-the arrangement and design of the electricalcomponents and terminations so as to avoid electrical breakdown, forexample by sparking across an airgap between capacitor terminals.

A further important feature of the invention is that the edges of theencompassing screen are rolled over so that all outwardly facingsurfaces of the screen are smoothly curved and of large enough radiusfor meeting the stressing requirement between adjacent screens ofadjacent modular units when mounted in a stack. It will be appreciatedthat by encompassing each module with such a screen, the surfaces ofwhich all have very small curvature compared with that of, for example,capacitor terminals, the stressing problems between one module and thenext are greatly eased. Furthermore, this feature eases the stressingproblem between that module of the stack at the highest voltage andnearby earthed objects, such as a main enclosure within which the stackis mounted.

The invention also provides a modular unit as aforesaid in combinationwith an inverter for supplying power to the unit, which invertercomprises two thyristors and means for switching the thyristorsalternately on and off, the thyristors being connected for switchingcurrent from a source, or sources, of direct current to provide analternating current to an output load, a resonant circuit connected toreceive alternating current from the thyristors, which resonant circuitis connected and arranged to so resonate that current reversal in theresonant circuit causes cutoff of that thyristor which is on before theother thyristor is triggered on, two unidirectional overflow currentpaths connected respectively across the thyristors to bleed off reversecurrent from the resonant circuit after this has cutoff the on thyristoras aforesaid, each said unidirectional bleed current path including aninductor.

The two unidirectional bleed current paths may share a common inductor.

2 BRIEF DESCRIPTION OF THE DRAWINGS Specific constructions of apparatusembodying the invention will now be described by way of example and withreference to the accompanying drawings in which:

FIG. 1 is a side view partly sectioned in the direction of arrow A inFIG. 2,

FIG. 2 is a plan view with some parts omitted,

FIG. 3 is a fragmentary view of part of the apparatus as seen in thedirection of arrow B in FIG. 2,

FIG. 4 is a circuit diagram of the electrical apparatus shown in FIGS. 1to 3,

FIG. 5 is an electrical circuit diagram of part of an inverter,

FIG. 6 is an electrical circuit diagram of the part of the invertershown in FIG. 5, with a component omitted,

FIG. 7 shows graphs of waveforms with time of voltages and currents atvarious points marked in FIG. 5,

FIG. 8 is an electrical circuit diagram of part of FIG. 5 showing theload circuit in more detail,

FIGS. 9a and 9b show waveforms associated with the description of theoperation of the circuit shown in FIG. 8, and

FIG. 10 shows a modification of the inverter shown in FIG.

DESCRIPTION OF PREFERRED EMBODIMENTS In this example, a generator ofhigh electrical voltage is provided by three modular units connected ina stack.

The basic modular unit 11, forming the central unit in this example,comprises four capacitors C1, C2, C3 and C4 connected with diodes D1,D2, D3, D4 in Cockcrofi-Walton configuration as may be seen best fromFIG. 4. Resistors R1, R2, R3 and R4, each of I00 megohms, are providedto allow the charge on the capacitors to leak away slowly so that a highvoltage is not maintained across the apparatus while it is not switchedon.

The lower unit lli and the upper unit 110 are basically similar to theunit 11 but have slight modifications so that they may conveniently formrespectively an input unit and an output unit.

The units lli and 110 have the same array of capacitors C1, C2, C3, C4and diodes D1, D2, D3, D4 and resistors R1, R2, R3, R4. However, theinput unit 1 1i is provided with a connection 12 to a socket PLl towhich input alternating current supply is connected. The input unit lliis also provided with a 220 ohm 3 watt resistor R5, and diode andcapacitor D5 and C5 respectively for monitoring purposes. The resistorR5 is a shunt resistor arranged so that measurement of the voltagedeveloped across the resistor R5 provides an indication of the outputcurrent. The arrangement of diode D5 and capacitor C5 provides formeasuring the leakage current through the resistors R1 and R2, fromwhich an indication of the voltage is derived.

The output unit 110 has a cable connection 13 to an output socket 8K2.Incorporated in this cable are a series of resistors R6, R7, R8, R9,each of 2.2 kilohms and 6 watts rating, The

purpose of these resistors is to limit the current drawn from theoutput, in particular in the event that this should be shorted.

FIGS. 1, 2 and 3 show the spacial disposition of the components. In eachmodular unit 11, or or 1 1:, the capacitors C1, C2, C3 and C4 aremounted in pairs on spaced plates 14, 15 provided with brackets l6, l7,l8, 19 adapted for attachment to perspex rod supports 21, 22.

Each modular unit has input and output terminals, of which only one ofeach can be seen in FIG. 1. In the stack, the appropriate input andoutput terminals are cross-connected by wire link connectors 23.

Each modular unit is encompassed by a cylindrical electricallyconducting screen 24, the edges of which are rolled as at 25 to avoidthe formation of high electric field stress.

The cylindrical screen 240 for the output modular unit 110 is shaped atthe top to receive a smoothly-curved, tight-fitting lid 26 ofelectrically conducting material.

In this example, the modular units are each designed to produce 20kilovolts. The components within the modular units are arranged to avoidelectrical breakdown in air for a 20 kilovolts differential betweeninput and output terminals. The screen 24 is held at a potential midwaybetween the input and output terminals and is stressed for a 10 kilovoltdifierential in air between these terminals and the screen.

In the stack, the screens 24 are stressed for a 20 kilovolt differentialbetween adjacent screens and the mounting within the main enclosure,such as indicated at 27, has to be such that no breakdown occurs betweenthis and any of the screens 24, but particularly the screen 240 of theoutput modular unit as this will be at the highest potential relative tothe earth support 27.

It will be appreciated that the stressing problems as between onemodular unit and the next, or as between one of the modular units andthe main supports, is greatly eased by the provision of the screens 24,the outwardly facing surfaces of which have very small curvaturecompared with, for example, the capacitor terminals.

A generator for a very high voltage may be constructed by adding modularunits 11 into the stack between the input and output modular units. Thestressing problems of such a construction are greatly reduced, sinceeach modular unit is internally ready stressed and only stressingbetween one module and the next and with the surroundings has to beconsidered.

Component values, or specification, not already given, are as follow:

Resistors R1 and R2 in the input modular unit 111' marked in FIG. 4 tohave a combined resistance of 50 megohms :2 percent.

Capacitors C1, C2, C3, C40.05 microfarads, l kilovolts working voltage,

Capacitor CS- [00 picofarads.

Diodes D1, D2, D3, D4 for basic modular units 11 and output units 1l0-LC 180.

Diodes D1, D2, D3, D4 for input modular units lli-EDl type RTD14.

Diode D5-2A 100F.

Monitor socket SKI=four-way Plessey Mk. IV socket.

FIG. 4 is drawn for a negative output stack. For positive output, alldiodes have to be reversed.

FIG. 5 illustrates part of an inverter for supplying alternating currentto the input at PLl of the Cockcroft-Walton high voltage generator ofFIGS. 1 to 4.

The output is taken, effectively from the point marked Vd, from across aload represented by resistor RL in FIG. 5 and shown in more detailedform within the dotted box in FIG. 8.

A direct current source provides voltages at +E, -E and 0 volts at thepoints marked appropriately in FIG. 5. Current is switched throughresistor RL from +E and E alternately by silicon-controlled rectifiersCSR] and CSR2. Triggering pulses for these thyristors are supplied froman external circuit not shown. Commutation is secured by a resonantcircuit comprising inductor LI and capacitor CCl. Diodes DDI and DD2drain reverse current after their associated thyristor, respectivelyCSRl and CSR2, has switched off.

An inductor L2 performs an important function which may best beunderstood by considering the operation of the circuit in the absence ofinductor L2, that is the circuit illustrated in FIG. 6.

Assuming the output from point A, FIG. 6, is a square wave, then justbefore switching on thyristor CSRl, the point- A will be at E volts anddiode DD2 will be conducting as indicated by the arrow. When thyristorCSR] is triggered, the potential at point A rises to +5 volts, thecurrent in the load resistor RL reverses and the current flowing throughthyristor CSRl into the resonant circuit comprising inductor L1 andcapacitor CCl aids the initial current. Half a cycle later the currentin the resonant circuit reverses and when this current exceeds that inthe load resistor RL, then thyristor CSRI cuts off and the surpluscurrent from the resonant circuit is returned to +E through the diodeDDI.

For this operation, the resonant frequency of the resonant circuit hasto be slightly higher than the switching frequency of the thyristors andthe characteristic impedance Ll/CCl) is made to be of the same order asRL so that the peak current in the commutating resonant circuit isgreater than the current in the load resistor RL.

This circuit, as shown in FIG. 6, suffers from a number of limitations:

a. In order that the thyristors should rapidly revert to the blockingstate, a substantial maintained reverse voltage should be applied. Inthis circuit, the voltage is limited to of the order of 1 volt by theparallel diode. Consequently the turn-off time of the thyristors isincreased and the maximum operating frequency of the inverter islimited.

b. If the load resistor RL falls below the value approximately Ll/CCl,commutation ceases with the result that both thyristors are switched onsimultaneously, thereby placing a short circuit directly across thesupply.

0. The inverter of this example drives a Cockcroft-Walton generator viaa stepup transformer. Under no-load conditions with square wave drivethe overshoot at the transformer secondary due to its leakage inductanceand stray capacities will be equal to the drive voltage. Since thenoload output of the Cockcroft-Walton generator is propor tional to thepeak-to-peak input voltage, excessive output voltages will generate withpossible disastrous consequences. It is therefore desirable that theedges of the driving waveform should be slowed up to minimize theovershoot.

It will be seen that the diodes DDI and DD2 are connected so as toprovide unidirectional overflow current paths connected respectivelyacross the thyristors CSRl and CSR2 to bleed ofi" reverse current fromthe resonance circuit afier this reverse current has cutoff the onthyristor. By incorporating an inductor in each of the overflow currentpaths, the abovementioned limitations may be avoided or reduced.

FIG. 5 illustrates the incorporation of an inductor L2 which is sharedby the overflow current paths of diodes DDI and DD2. overshoot Theoperation of the circuit of FIG. 5 is as follows:

Just before thyristor CSRI is triggered on, diode DD2 is conducting anda current Ic flows through the inductor L2 in the direction indicated byarrow Q. When thyristor CSRI is triggered on, a current Ie flows throughthe thyristor into the resonant circuit comprised by inductor L1 andcapacitor CCl. This current is into the resonant circuit is aided by thecurrent Ic, the sum being a current lb. The current Ic is falling inmagnitude whilst the current Ie is building up resonantly. The currentlc falls to a level where the diode DD2 comes nonconducting and then thepotential Vd at the output point (see the reference Vd on FIG. 5) beginsto rise resonantly from E volts to +E volts, the resonant circuit forthis voltage rise being provided by inductor L2 and the stray capacityacross the load resistor RL.

The waveforms of the various currents and voltages discussed above areshown in FIG. 7, the top waveform being that of the potential Vaappearing at the junction between inductors L1 and L2 (see FIG. 5).

The current lb in the resonant circuit of inductor L1 and capacitor CClreverses, the reversed current flowing through inductor L2. At time :1(see FIG. 7), the current lb is equal in magnitude but opposite indirection to the current lc so that current through the thyristor CSRlis zero and the thyristor CSRl ceases to conduct. As a consequence, avoltage L2dl b/dt is generated across the thyristor CSRl. The efiect ofthis may be seen as the spike in the waveform of the potential Va (FIG.7). This voltage spike generated by the inductor L2 rapidly removes thestored charge from thyristor CSRl and allows it to revert to theblocking condition. Shortly after this thyristor CSR2 it fired, theoperation being equivalent in the opposite direction.

Thus it will be seen that a large reverse voltage (Va) has been producedto speed up the recovery time the thyristors.

and the edges of the waveform applied to the load RL have been slowedup. Further if R1 is short-circuited, the inductor L2 appears as aninductive load and the values of inductor Ll, capacitor CCl and inductorL2 can be choun so that the circuit will continue to commutate even ifload resistor RL is short-circuited.

In order to consider the effect of the inductor L2 on the overshoot atthe secondary of the output transformer, where the load RL is providedby a transformer, reference is made to FIG. 8.

In FIG. 8, the transfonner T is shown connected in place of resistor RL,some parts of the remaining circuitry being omitted. Inductor L3 andcapacitor CC3 represent the leakage inductance and stray capacity of thetransfonner referred to the primary winding.

n application of a voltage step 215 volts to the transformer input, thepoint reference C rises resonantly to 4B volts. Just before diode DDlconducts, the point B is at +15 volts, that is the voltage acrossinductor L2 is zero. Hence the rate of change of current in the inductorL2 is zero and the voltages at points C and D are also at +5 volts andthe current at this instant is 2E.CC3/ L2+L3.

When diode DDl conducts the efl'ective series inductance falls to L3.Under these conditions it can be shown that the magnitude of theovershoot is 25 L3/L2+L3.

The effect of this is illustrated in FIGS. 9(a) and 9(b1. FlG. 9(a)shows the voltage waveform across the transformer when L2=0, that iswith the FIG. 6 configuration. FIG. 9(1)) shows the voltage waveformacross the transfonner when an inductor L2=8L3 is incorporated as in theFIG. 5 configuration.

FIG. 10 illustrates the modification of the inverter in which the loadresistor RL is connected across capacitor CCl' forming part of theresonant commutation circuit. A single inductor Ll with three tapsperforms the function of inductors L1 and L2 of FIG. 5. Thyristors CSRland CSRZ', and the diodes DD! and DD2' have an equivalent arrangementand function to the corresponding components in FIG. 5.

The invention is not restricted to the details of the foregoingexamples.

I claim:

1. a modular unit for a high voltage generator of the Cockcroft-Waltontype, comprising one or more Cockcroft- Walton stages mounted within anelectrically conducting casing, which fonns an encompassing screen forthe said stages,

and input and output terminals for connection, as desired, to anothermodular unit so that the voltage step-up of each unit is added, wherebya stack of N interconnected modular units can generate a voltage NV,where V is the voltage generated by each modular unit on its own.

2. A modular unit as claimed in claim 1, wherein the said screen is heldat a potential intermediate that of the input and output terminals ofthe modular unit.

3. A modular unit as claimed in claim 2, wherein the said screen is heldat a potential midway between that of the input and output tenninals ofthe modular unit.

4. A modular unit as claimed in claim 1, wherein each of the said stageswithin the modular unit is stressed for predetermined potentialdifferences between its input and output terminals and between theseterminals and the encompassing screen.

5. A modular unit as claimed in claim 4, which unit together with itsencompassing screen is stressed for a predetermined potential differencebetween it and an adjacent modular unit in a stack of modular units.

6. A modular unit as claimed in claim 5, wherein the edges of theencompassing screen are rolled over so that all outwardly facingsurfaces of the screen are smoothly curved, the radius of curvaturebeing large enough for meeting the stressing requirement betweenadjacent screens of adjacent modular units when mounted in a stack.

7. A modular unit as claimed in claim 1, in combination with an inverterfor supplying power to the unit, which inverter comfirises twothyristors and means for switching the thyristors ternately on and off,the thyristors being connected for switching current from a source, orsources, of direct current to provide an alternating current to anoutput load, a resonant circuit connected to receive alternating currentfrom the thyristors, current reversal in the resonant circuit causingcutofi' of that thyristor which is on before the other thyristor isswitched on, two unidirectional overflow current paths connectedrespectively across the thyristors to bleed off reverse current from theresonant circuit after this has cut off the on thyristor as aforesaid,each said unidirectional bleed current path including an inductor.

8. A modular unit in combination with an inverter as claimed in claim 7,wherein the two unidirectional bleed current paths share a commoninductor.

1. A MODULAR UNIT FOR A HIGH VOLTAGE GENERATOR OF THE CockcroftWaltontype, comprising one or more Cockcroft-Walton stages mounted within anelectrically conducting casing, which forms an encompassing screen forthe said stages, and input and output terminals for connection, asdesired, to another modular unit so that the voltage step-up of eachunit is added, whereby a stack of N interconnected modular units cangenerate a voltage NV, where V is the voltage generated by each modularunit on its own.
 2. A modular unit as claimed in claim 1, wherein thesaid screen is held at a potential intermediate that of the input andoutput terminals of the modular unit.
 3. A modular unit as claimed inclaim 2, wherein the said screen is held at a potential midway betweenthat of the input and output terminals of the modular unit.
 4. A modularunit as claimed in claim 1, wherein each of the said stages within themodular unit is stressed for predetermined potential differences betweenits input and output terminals and between these terminals and theencompassing screen.
 5. A modular unit as claimed in claim 4, which unittogether with its encompassing screen is stressed for a predeterminedpotential difference between it and an adjacent modular unit in a stackof modular units.
 6. A modular unit as claimed in claim 5, wherein theedges of the encompassing screen are rolled over so that all outwardlyfacing surfaces of the screen are smoothly curved, the radius ofcurvature being large enough for meeting the stressing requirementbetween adjacent screens of adjacent modular units when mounted in astack.
 7. A modular unit as claimed in claim 1, in combination with aninverter for supplying power to the unit, which inverter comprises twothyristors and means for switching the thyristors alternately on andoff, the thyristors being connected for switching current from a source,or sources, of direct current to provide an alternating current to anoutput load, a resonant circuit connected to receive alternating currentfrom the thyristors, current reversal in the resonant circuit causingcutoff of that thyristor which is on before the other thyristor isswitched on, two unidirectional overflow current paths connectedrespectively across the thyristors to bleed off reverse current from theresonant circuit after this has cut off the ''''on'''' thyristor asaforesaid, each said unidirectional bleed current path including aninductor.
 8. A modular unit in combination with an inverter as claimedin claim 7, wherein the two unidirectional bleed current paths share acommon inductor.